Swirls of blue in the Great Lakes mark locations of recent autumn storms whose winds have stirred up sediment in the lakes. The silt and quartz sand acts as a tracer particle, making visible the circulation patterns of the lakes. In contrast, the green streaks mark locations of calmer winds and warmer temperatures where algae blooms have grown. Note the fundamental dissimilarity in their structures. Blue eddies turn over and mix in a fashion reminiscent of convective instabilities while the green blooms are far more uniform in structure. #
Tag: fluid dynamics
The Coalescence Cascade
When a droplet impacts a pool at low speed, a layer of air trapped beneath the droplet can often prevent it from immediately coalescing into the pool. As that air layer drains away, surface tension pulls some of the droplet’s mass into the pool while a smaller droplet is ejected. When it bounces off the surface of the water, the process is repeated and the droplet grows smaller and smaller until surface tension is able to completely absorb it into the pool. This process is called the coalescence cascade.
Coughing Contagions
Schlieren imaging has applications even in public health. This video demonstrates the spread of contagion via coughing with and without a mask on. Although air from the cougher’s lungs escapes the sides of the mask, it mostly rises on a thermal plume rather than projecting 1 to 2 meters forward in a turbulent jet as in the maskless case. Flu season is just starting. Don’t forget to get your flu shot!
High-Altitude Balloon Flight
Tangentially fluids-related, but SpaceWeather has a fun video of a high-altitude helium balloon bursting. Although this balloon carried a space-related payload, it’s the same type of set-up used for weather balloons. With only a few basic assumptions, it’s possible to do some neat calculations on the buoyancy, loading capacity, and behavior of such balloons.
Vortex Street Sim
This numerical simulation shows a von Karman vortex street in the wake of a bluff body. As flow moves over the object, vortices are periodically shed off the object’s upper and lower surfaces at a steady frequency related to the velocity of the flow. The simulation takes place in a channel; note how the thickness of the boundary layers on the walls increases with downstream distance, forcing a slight constriction on the vortex street in the freestream.
Soap Film Flow Viz
Flowing soap films provide an educational and beautiful method for visualizing the wakes of objects in two-dimensional flows. High-speed photography highlights the interference patterns on the soap film, providing detail without the necessity for the particulate tracking of other flow visualization methods. Highlights here include wakes behind bluff bodies, interacting cylinders, and flapping flags. (pdf) #
Aurora from the ISS
The solar wind, a rarefied stream of hot plasma ejected from the sun, constantly bombards Earth’s magnetic field. This results in the formation of the magnetosphere, which deflects most of these charged particles away from the earth. Some of them, however, are drawn toward the magnetic poles; when these charged particles strike the upper atmosphere, they cause the gases there to release photons, resulting in the lights we know as auroras. This animation shows the International Space Station flying through the aurora australis–the southern lights. The fluid-like motion of the aurora is no accident; though diffuse, the solar wind is still a fluid governed by magnetohydrodynamics.
Sharkskin-Style Swimsuits
Fans of swimming will recall the controversies of the now-banned sharkskin-style swimsuits that helped break so many records in the past few years. The suits decrease drag on a swimmer both by making them more hydrodynamic in form and by drastically reducing skin friction where the water meets the swimmer’s body. In addition to decreasing the two major sources of drag on a swimmer, the compression provided by the material can help increase blood flow to muscles. These improvements came at a high material cost, though, and, since the technology was not viable for all athletes, it has since been banned.
Mixing in Space
Living here on earth, we are so accustomed to gravity’s effects on fluid behaviors that it’s not always obvious how microgravity will affect them. Here astronaut Richard Garriott demonstrates mixing and separating immiscible liquids in space.
Flow Vis
Place a viscous fluid in the gap between two plates of glass and you have created a Hele Shaw cell. If a less viscous fluid is then injected between the plates, a fascinating pattern of finger-like protrusions results. This is known as the Saffman-Taylor instability. Because of the relative simplicity of the set-up, it’s possible to create such experiments at home using common household fluids like glycerin, dish soap, dyed water, or laundry detergent. (Photo credits: Jessica Rosencranz, Jessica Todd, Laurel Swift et al, Andrea Fabri et al, Tanner Ladtkow et al, Mike Demmons et al, Trisha Harrison, Justin Cohee, and Erik Hansen)
